The present invention relates to an automatic adjuster for internal shoe drum brakes with duo-servo action.
Duo-servo brakes which automatically adjust the clearance between brake shoes and a drum during the course of wear of the brake shoe surface during operation are already known.
UK Patent Specification No. 1,081,441, describes a construction in which the adjusting device comprises a housing which is movable against frictional resistance upon a back plate during brake operation, the housing carrying a screw-threaded thrust member disposed between opposed ends of the brake shoes, a single spring-loaded lever being actuable to adjust the thrust member by movement of the member, caused by drum rotation, first in one direction and then in the opposite direction and adjustment being accompanied by limited movement of the housing on the back plate. It will be appreciated that the effectiveness of this construction depends upon rotation of the drum in both directions and upon housing movement on the back plate.
UK Patent Specification No. 1459872, on the other hand, describes a construction having a support which is fixed to the back plate. A pair of co-axial screw-threaded members and nuts are slidably mounted in a common sleeve mounted in the support, the screw-threaded members engaging opposed brake shoe ends so that rotation in one direction of the nuts on their respective screw-threaded members causes an effective increase in their overall length and an adjustment to the brake shoe clearance from the associated drum. A common single lever is engageable with ratchet wheels, one for each nut, the lever being rotatable about one or other of two pivot pins, disposed at the ends of arcuate slots formed in the lever, to effect rotation of a ratchet wheel and thus cause adjustment of one or the other nut. There is a consequent movement of the pins in the arcuate slots which complicates the construction and can lead to unsatisfactory operation.
It is an object of the invention to provide a new and improved automatic adjuster of simple and robust design which operates automatically to adjust each brake shoe so as to compensate for wear which otherwise would cause excessive clearance between the shoes and the associated brake drum.